Lightning current transfer arrangement of a wind turbine

ABSTRACT

A lightning current transfer arrangement for a wind turbine is provided, to enable lightning current and electrostatic discharge current to be discharged from a first and a second part of the wind turbine being rotatable relative to each other. The lightning current transfer arrangement is arranged to provide electrical contact between the first and second parts, and has a current transfer unit electrically coupled to the first part, and an electrically conductive slideway electrically coupled to the second part, and rotatable relative to the current transfer unit. The current transfer unit further has a slider elastically biased towards the slideway, where the slider comprises a slide piece made of conductive plastics and contacting the slideway, and an electrode made of metal and directed towards the slideway, the electrode and the slide piece being rigidly coupled.

FIELD OF THE INVENTION

The present invention relates to a lightning current transferarrangement of a wind turbine, and to a wind turbine comprising one ormore lightning current transfer arrangements.

BACKGROUND OF THE INVENTION

In a known lightning conductor system for a wind turbine a gap in thelightning down-conductor between a pitchable rotor blade and the windturbine's nacelle—two parts which are rotatable relative to eachother—is bridged by a lightning current transfer arrangement havingsliding contacts which contact complementary slideways, as described inWO 2005/050008 A1. The sliding contacts include a contact rod whichprovides the electric contact with the slideway, and a slide padsurrounding the contact rod to increase the sliding surface and lowersliding friction.

Other types of sliding contacts for wind turbines with in the form ofmetal or carbon brushes are known, e.g. from WO 2004/001224 A1.

U.S. Pat. No. 7,390,169 B2 describes a lightning current transferarrangement for a wind turbine having a sliding contact and a spark gaptowards a contact member. The sliding contact and the spark gap areconnected in parallel.

SUMMARY OF THE INVENTION

A lightning current transfer arrangement of a wind turbine is providedin one embodiment. The wind turbine comprises a first and a second partbeing rotatable relative to each other. The lightning current transferarrangement is arranged to provide electrical contact between the firstand second parts. It comprises a current transfer unit electricallycoupled to the first part, and an electrically conductive slidewayelectrically coupled to the second part, and rotatable relative to thecurrent transfer unit. The current transfer unit further comprises aslider elastically biased towards the slideway, where the slidercomprises a slide piece which is made of conductive plastics andcontacts the slideway, and an electrode which is made of metal and isdirected towards the slideway. The electrode and the slide piece arerigidly coupled.

According to another aspect a wind turbine is provided comprising atleast one lightning current transfer arrangement. The wind turbinecomprises a first and a second part being rotatable relative to eachother. The at least one lightning current transfer arrangement isarranged to provide electrical contact between the first and secondparts. It comprises a current transfer unit electrically coupled to thefirst part, and an electrically conductive slideway electrically coupledto the second part, and rotatable relative to the current transfer unit.The current transfer unit further comprises a slider elastically biasedtowards the slideway, where the slider comprises a slide piece which ismade of conductive plastics and contacts the slideway, and an electrodewhich is made of metal and is directed towards the slideway. Theelectrode and the slide piece are rigidly coupled.

Other features are inherent in the methods and products disclosed orwill become apparent to those skilled in the art from the followingdescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are explained by way of examplewith reference to the accompanying drawings. It is to be noted that theappended drawings illustrate only examples of embodiments of thisinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1 illustrates an embodiment of a wind turbine with wind turbineblades and a lightning grounding system;

FIGS. 2a and 2b illustrate the hub portion of the wind turbine of FIG.1, including the charge transfer region, and a first embodiment of alightning current transfer arrangement;

FIG. 3 illustrates a first embodiment of a current transfer unit of thelightning current transfer arrangement according to FIGS. 2a and 2 b;

FIGS. 4a and 4b are perspective front and rear views of an embodiment ofa slider with an electrode and a slide piece, used in FIG. 3;

FIG. 5 illustrates another embodiment of a lightning current transferarrangement;

FIGS. 6a and 6b provide a side view and a perspective view of anembodiment of a slider mounted on a flat strip, with an electrode formedas a spark gap, and a slide piece, used in FIG. 5; and

FIG. 7 is a side view of an alternative embodiment of a slider and anelectrode forming two spark-gap electrodes.

The drawings and the description of the drawings are embodiments of theinvention and not the invention itself.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, reference is made to embodiments of the invention.However, it should be understood that the invention is not limited tospecific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practice theinvention.

Furthermore, in various embodiments the invention provides numerousadvantages over the prior art. However, although embodiments of theinvention may achieve advantages over other possible solutions and/orover the prior art, whether or not a particular advantage is achieved bya given embodiment is not limiting of the invention. Thus, the followingaspects, features, embodiments and advantages are merely illustrativeand are not considered elements or limitations of the appended claimsexcept where explicitly recited in a claim(s). Likewise, reference to“the invention” shall not be construed as a generalization of anyinventive subject matter disclosed herein and shall not be considered tobe an element or limitation of the appended claims except whereexplicitly recited in a claim(s).

In embodiments of a lightning conductor system for a wind turbine, thegap between the rotor blade and the nacelle, as both parts are rotatablerelative to each other, is bridged by a lightning current transferarrangement that transfers the lightning current directly from the rootof the rotor blade to the nacelle. Moreover, static charges arising dueto friction of the air with the rotor blades are discharged, as thelightning current transfer arrangement provides ground contact.

A conventional sliding contact arrangement, as known from WO 2005/050008A1 mentioned at the outset, includes a metallic contact rod acting as acontact slider encircled by a slide piece made of electricallynon-conducting low-friction plastics material.

It has been observed in the context of the present invention that inconventional sliding contact arrangements the galvanic connectionbetween the contact slider and the slideway may be interrupted for shorttime intervals. This might be due to the contact slider lifting off theslideway, thereby momentarily disconnecting and connecting again thecurrent path. Due to the inductance inherent in the total current pathfrom the rotor blade tip to ground, and due to accumulation of electriccharges in the rotor blades, such interruption of the current flow caninduce a relatively high voltage which, in turn, may give rise tosparking flashovers at the sliding contacts. Therefore, sudden changesin the current flow between the rotor blade tip and ground may arise,producing unwanted electromagnetic radiation with a high bandwidth,radiated by the wind-turbine blades acting as a big antenna. Moreover,wind turbines located near a strong radio transmitter may receive andre-transmit a considerable amount of radio power from the radiotransmitter. Here, the wind turbine can act as a sort of transponderwhich receives electromagnetic energy in a narrow radio frequency band.This may even increase the frequency and intensity of sparkingflashovers at the sliding contacts, resulting in a retransmission of asignificant part of received radio power in a broad frequency band.

It has further been observed that sparking flashovers result inincreased wear of the slider due to spark erosion.

In a conventional sliding contact arrangement, sparking flashovers mayoften arise as the slide piece reduces the force at which theelectrically conducting part of the contact arrangement, i.e. theinner-end side of the electrode is in galvanic contact with thecomplementary contact area.

The embodiments described herein relate to a lightning current transferarrangement that is arranged to provide electrical contact between firstand second parts of a wind turbine being rotatable relative to eachother, for example, a rotor blade and the nacelle.

In the embodiments, a slide piece made is made of an electricallyconducting plastics material. It has been recognized by the inventorsthat the slide piece provides consistent mechanical contact to theslideway, and therefore the slide piece is in consistent electricalcontact as well. As reliability of electric contact is improved when theslider is moving over the slideway, electromagnetic radiation of thewind turbine due to sparking flashovers may be reduced.

The embodiments described herein comprise an electrically conductiveslideway and a current transfer unit with a slider elastically biasedtowards the slideway. The slider comprises a metal electrode, which isoriented towards the slideway, and a slide piece made of conductiveplastics, where the electrode and the slide piece are rigidly coupled toform one unit in which the slide piece is contacting the slideway.

The inner-end surface of the electrode and the sliding face of the slidepiece jointly establish a contact area where the rear end of theelectrode provides electric contact to the contact surface of thecomplementary contact area and the slide piece ensures low-frictionslide on the slideway. Furthermore, the slide piece stabilizes thesliding of the electrode over the complementary contact surface ensuringthat the electrode is substantially perpendicular to the complementarycontact surface. The sliding surface of the slide piece will normally besignificantly larger than the sliding end surface of the electrode.

With the embodiments described here, wear of the slide piece may bereduced because of two effects. Firstly, spark erosion may be reduceddue to the improved electrical contact of the slider. Secondly, themechanical bias necessary to provide reliable electrical contact of theslider to the slideway can be lowered compared to conventional sliders,resulting in reduced wear of the slider.

The term “conductive plastics” may include electrically resistivematerials or semiconducting materials. Electrically conducting plasticmaterials exhibit a conductivity of more than 10⁻⁷ S/m. Typically,conductivity of these conductive plastics materials is in a range fromabout 10⁻⁶ S/m to about 10⁵ S/m.

In some embodiments, the conductive plastics material has a matrix thatis isolating. However, the material includes additives that provideelectrical conductivity. Plastics materials especially suitable as rawmaterials in terms of electric and mechanic properties and durabilityare, for example, polyamide (Nylon®), polyoxymethylenes (POMs),Vesconite®, ultra high molecular weight polyethylene (UHMWPE). In someembodiments, the material includes electrically conductive additives,such as carbon black, graphite, metal flakes, metal fibers, particles orfibers coated with metal, conductively coated minerals, conductivelycoated ceramics, and pulverised anthracite.

In some embodiments, the material of the slide piece has a matrix of aconductive plastics material. In this case, conductivity is achieved inthe material itself. However, in some embodiments, conductivity of amaterial with a conductive matrix is enhanced by means of additives, asmentioned above. Conductive plastics materials include polyacetylene(PAC), polyanilin (PANI), polyparaphenylene (PPP), polypyrrole (PPy) andpolythiophene (PT).

During lightning strikes high electric current strengths may occur thatcan damage the slide piece. Therefore, in some embodiments, theelectrode is arranged to also contact the slideway. In theseembodiments, electrical contact is provided by the slide piece and theelectrode as well. Therefore, during lightning strikes, the major partof the lightning current will flow directly from the electrode to theslideway to ground, bypassing the slide piece. In this way, thelightning current transfer arrangement is therefore arranged towithstand the high current and thermal stress that typically occursduring a lightning strike.

In some embodiments of the slider, the electrode and the slide piecehave sliding surfaces, and the sliding surfaces of the slide piece andof the electrode are co-planar to provide electrical contact of theelectrode in case of lightning strikes. However, when no lightningstrike occurs, the slide piece may carry a significant part ofelectrostatic discharge current. Therefore, should there be shortinterruptions of the electric connection between the metallic electrodeand the slideway, discharge current can still flow through the slidepiece.

Due to different sliding friction coefficients of the sliding piecematerial and the electrode material with respect to the slideway,different mechanical forces will act upon the electrode and the slidepiece part of the slider, respectively. Some parts of the interfacebetween both materials may therefore be under tensile stress. Toincrease mechanical stability of the slider, in some embodiments, theelectrode's sliding surface is circumscribed by the slide piece'ssliding surface. In this way, a robust construction of the combinationof the electrode and the sliding piece is achieved.

In some embodiments, only the slide piece is in mechanical contact withthe slideway. In combination with the material of the slideway surface,the wear-pad material provides a low sliding friction coefficient andtherefore low wear. In these embodiments, the electrode is arranged tomove over the slideway at a distance. As the slide piece is made ofconductive plastics, a permanent current path for the discharge ofelectrostatic charges is provided, even when the spark gap is not lit.However, to protect the slider from damage by lightning currents, theelectrode is arranged to move over the slideway at a distance, therebyforming a spark gap to the slideway. In some embodiments, the slidepiece and the electrode are electrically connected in parallel.Therefore, the slide piece contacting the slideway provides a firstcurrent path to the slideway, and the electrode provides a secondcurrent path to the slideway upon occurrence of a spark bridging thespark gap, where the first and second current paths are parallel currentpaths.

When the spark gap is not lit, electrostatic charges are discharged overthe first current path, and the entire discharge current flows over theelectrically conducting slide piece. It may therefore be sufficient toarrange the slide piece to carry the current arising from electrostaticdischarges. However, in case of a lightning strike, the electric voltagedrop of the slide piece and the sliding connection of the slide piecewith the slideway will be sufficient to fire the spark gap. As theresistance of the electric arc of the spark gap is much lower than theresistance of the sliding piece contacting the slideway, the major partof the lightning current will flow over the electric arc of the sparkgap. Therefore, the slide piece is protected from damage due tolightning strikes currents.

To provide a spark gap to the electrical conducting slideway, in someembodiments, the electrode is recessed from the sliding surface of theslide piece by a recess distance. Therefore, as the slide piece followsthe slideway, the distance between the spark-gap electrode and theslideway is kept constant at about the recess distance. In theseembodiments, the slide piece has two functions: firstly to providereliable electric contact to the electrically conducting slideway,secondly, to act as a spacer for the spark gap. The spark gap electrodemoves in unison with the slide piece, controlling the width of the sparkgap. If the side piece moves up or down due to radial or axialunevenness of the slideway, the spark gap electrode also moves up ordown to the same extend. By this way, a constant width of the spark gapelectrode is provided, therefore, the firing voltage of the spark gapcan be controlled. The firing voltage corresponds to the maximum voltagedrop through the slide piece occurring when the wind turbine is struckby lightning. As the maximum voltage drop can be controlled by the sparkgap width, the maximum current that flows through the slide piece beforethe spark gap is lit can be controlled.

Protection of the slide piece by limiting the maximum current throughthe first path can be further enhanced when the first current path thatincludes the slide piece exhibits a further resistor. To this end, insome embodiments, the slide piece's resistance in the first current pathis from about 10 kΩ to about 10 MΩ. In these embodiments, the plasticsmaterial of the slide piece not only acts as a contact slider and aspacer for the spark gap, but also as a resistor for protection of theslide piece, further limiting the current strength in the slide piece,for example during lightning strikes.

However, as a further function, the lightning grounding system of thewind turbine is made high-ohmic, reducing the inclination of the windturbine to act as an antenna that radiates unwanted electromagneticwaves. For example, the resistance in the permanent discharge pathfurther helps to reduce the magnitudes of sudden current changes thatarise due to short interruptions of the electric connection or due toreception of radio power originating of nearby radio transmitters.

So far, the focus has been on the functionality of contact sliders. Insome embodiments the lightning current transfer arrangement not only hasone slider and one associated slideway, but two sliders with twoassociated slideways. This is useful, for example, if the first andsecond parts are rotatable relative to each other around two differentrotation axes. The first slideway is electrically coupled to the secondpart, and the second slideway is electrically coupled to the first part.The second slider provides electrical contact to the second slideway,where its slide piece contacts the slideway and an electrode towards theslideway. The two sliders are coupled end-to-end in series. In someembodiments both sliders are of the spark-gap type with a high-ohmicslide piece. However, in other embodiments only the first slider is ofthe spark-gap type with a high-ohmic slide piece, while the secondslider has an electrode that also contacts the slideway, and therebyprovides low-ohmic electric contact between the first part and thespark-gap electrode residing with the first slider.

In some embodiments, the lightning current transfer arrangement with twosliders and two slideways is arranged between the rotor blade and thenacelle of the wind turbine. Such a lightning current transferarrangement enables charges to be discharged from the rotor blade toground via the nacelle. The current transfer unit is mounted on the hub,and rotates with the hub. However, both sliders and a cable connectioncoupling them are electrically isolated from the hub, so that lightningcurrent bypasses the hub and rotor bearings.

The following is a detailed description of embodiments of the inventiondepicted in the accompanying drawings. The embodiments are examples andare in such detail as to clearly communicate the invention. However, theamount of detail offered is not intended to limit the anticipatedvariations of embodiments; but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims.

FIG. 1 shows an embodiment of a wind turbine 1 with a nacelle 3 mountedby means of an azimuth bearing on a tower 2. The wind turbine 1 hasthree rotor blades 5 mounted pitch-adjustably on a hub 4 by means ofblade (or pitch) bearings 11 (FIG. 2a ). The hub 4 is e.g. mounted on amain shaft 10 (FIG. 2a ) supported by main-shaft bearings in the nacelle3.

A grounding system protects the mechanic and electronic equipment fromdamage, e.g. by lightning strikes, and prevents the rotor blades 5 fromaccumulating static charges. The tips 8 of the wind turbine blades 5 areequipped with lightning receptors 6 electrically connected to the rootof the rotor blades 5 by a down conductor 9 leading internally along theblades 5. Further lightning receptors connected to the down conductor 9are distributed over the length of the blades 5. In some embodiments theblades 5 are partly covered with conducting meshes connected to the downconductor 9.

The rotor blade is made of electrical non-conducting material, such asglass fiber reinforced plastics. The receptors may not only be hit bylightning, but in certain conditions may also collect static charges.

FIGS. 2a and 2b illustrate a more detailed view of the hub regionincluding the hub 4, a blade bearing 11, a root part of a rotor blade 5and the adjacent part of the nacelle 3 of the wind turbine of FIG. 1,with a first embodiment of a lightning current transfer arrangement. Thehub 4 is fixed on the main shaft 10 and rotates with it. The main shaft10 rotates around a (nearly) horizontal axis, while the rotor blades 5are rotatable around (non-stationary) longitudinal axes which areperpendicular to the main-shaft axis. Thus, seen in the perspective ofan observer on the rotor, a rotor blade 5 and the nacelle 3 rotatearound two different (perpendicular) axes. A current transfer unit 13 ismounted on the hub 4 between the root of each blade 5 and the nacelle 3.It enables charges to be transferred directly from the blade 5 to thenacelle 3, thereby bypassing the hub 4. Each rotor blade 5 has aconductive slideway 18 arranged around its root and internally connectedto the down conductor 9. In embodiments where the blades 5 are made ofnon-conducting material, there is no direct electrical path designedbetween the blade slideway 18 and the hub 4. The side of the nacelle 3facing the hub 4 is equipped with a conductive slideway 17 around themain-shaft opening of the nacelle 3, which is electrically coupled to adown conductor on the exterior of the nacelle 3 and the tower 2 toground 12. What is called a “lightning current transfer arrangement” isformed by a current transfer unit 13, its associated blade slideway 18and the nacelle slideway 17. As a current transfer unit 13 is providedfor each rotor blade 5, the nacelle slideway 17 is shared between threelightning current transfer arrangements, and in particular, the threetransfer units 13.

The current transfer unit 13 of FIG. 3 has a base support 14 fixedlymounted to the rotor hub 4 by means of mounting bolts. Thus, the currenttransfer unit 13 rotates with the main shaft. The slider 15 is mountedto one end of an elastically deformable strip 23 of electricallyinsulating material which is—at the other end—mounted to a mountingflange of the base support 14. This mounting flange projects towards thenacelle 3, so that the deformable strip 23 is bent to elastically biasthe slider 15 towards the nacelle slideway 17.

In this embodiment, slider 15 is mounted in a mounting bracket withbolts in adjusting slots. Therefore, it can be pivoted around swivelaxis 25 into a position to apply flat to the nacelle slideway 17.

The slider 15 (see FIGS. 4a and 4b ) includes a slide piece 28 ofelectrically conductive plastics. The material of the slide piece 28 hasa matrix of isolating plastics. The material further includes a carbonblack additive to make it electrically conducting.

Slider 15 includes a metal electrode 32 that can, upon assembly, berigidly coupled with slide piece 28. In this embodiment, electrode 32 ismade of brass, and arranged to provide electrical contact to slide piece28 as well as slideway 17. In this embodiment, slide piece 28 has asliding surface 30, and electrode 32 has a sliding surface 33, whereinboth sliding surfaces are co-planar. Moreover, the sliding surface 33 ofthe electrode is circumscribed by the slide piece's sliding surface 30.

The blade slideway 18 is a part of the lightning current transferarrangement of FIG. 2b , and the current transfer unit 13 has a secondslider 16 contacting the blade slideway 18. In this embodiment, thesecond slider 16 is the same as described in relation with the slider ofFIGS. 4a and 4 b.

The slider 16 is mounted to the base support 14 in a mannercorresponding to that of slider 15: The slider 16 is pivotable around aswivel axis 26. It is mounted on an elastically deformed strip 24 madeof insulating material, the strip being mounted on an outwardlyprojecting flange of the base support 14. The slider 16 is therebyelastically biased towards the blade slideway 18.

The electrodes 32 of the two sliders 15, 16 are electrically coupled bya cable 19, thereby providing a current path from the blade 5 to thenacelle 3 and bypassing the hub 4.

FIG. 5 illustrates another embodiment of the lightning current transferarrangement with sliders 15, 16 contacting slideways 17, 18 of thenacelle 3 and the rotor blade 5, respectively. In this embodiment,slider 16, which is in contact with slideway 18 at the rotor blade side,is of the same kind as described in relation with the slider of FIGS. 4aand 4 b.

Moreover, similar to the current transfer unit of FIG. 3, the currenttransfer unit 13 has a base support 14 fixedly mounted to the rotor hub4. The slider 15 is mounted to one end of an elastically deformablestrip 23 which is bent such that it elastically biases the slider 15towards the nacelle slideway 17. In the same way, slider 16 is mountedto one end of an elastically deformable strip 24 which is bent such thatit elastically biases the slider 16 towards the rotor blade slideway 18.The electrodes 35, 32 of the two sliders 15, 16 are electrically coupledby a cable 19, thereby providing a current path from the blade 5 to thenacelle 3 and bypassing the hub 4.

The embodiment of FIG. 5 proposes a different coupling of the sliders15, 16 to the strips 23, 24, respectively. Instead of using mountingbrackets, sliders 15, 16 are mounted using an intermediate support 27(FIGS. 6a, 6b ) which is, in turn, coupled to the deformable strip 23,24 by means of snap-in coupling, which facilitates ease of removal.

Slider 15, in contact with nacelle slideway 17, differs from the sliderillustrated in FIGS. 4a and 4b , as will be explained in connection withFIGS. 6a and 6b . In this embodiment, electrically conductive slidingpiece 36 is the only part contacting slideway 17. The slide piece 36 ismade of electrically conductive plastics. The material of the slidepiece 36 has a matrix of isolating plastics, and including a carbonblack additive to make it electrically conducting. Moreover, in thisembodiment, the electrical resistance of slide piece 36 measured betweenelectrode 35 and slideway 17 is designed to be about 100 kΩ.

The electrode 35 is made of brass. However, the tip 20 of electrode 35is made of tungsten. As can be seen from FIG. 5, electrode 35 of slider15 is arranged to move over slideway 17 at a distance, withoutcontacting it, thereby tungsten tip 20 of electrode 35 and slideway 17forms a spark gap. Electrode 35 is recessed in the slider 15 by a recessdistance d. As electrode 35 and slide piece 36 are rigidly coupled,electrode tip 20 moves in unison with the slide piece 15 over thenacelle slideway 17, at distance w from the slideway surface (called the“spark-gap width”) controlled by the slide piece 36. The spark-gap widthw corresponds to the recess distance d. In this embodiment, slide piece36 and the electrode 35 provide parallel electrical paths. The slidepiece 36 contacting the slideway 17 provides a first current path to theslideway, and the electrode 35 provides a second current path to theslideway 17 upon occurrence of an electrical spark bridging the sparkgap, where the first and second current paths are parallel currentpaths.

In this embodiment, the tip 20 of electrode 35 is located within acavity 37 formed by the slide piece 36. The cavity has lateral reliefopenings which enable air to expand when abruptly heated by a spark. Therelief openings are oriented in parallel to the sliding direction (whichis perpendicular to the paper plane in FIG. 5). This orientation reducesthe occurrence of particles of material sputtered away from theelectrode tip 20 or the nacelle slideway 17, and blown outwardly in therelief openings, depositing on that surface region of the nacelleslideway 17 which are swept by slide piece 36.

FIG. 7 illustrates and alternative embodiment of a spark-gap type ofslider. In the embodiment, the relative arrangement of the slider 36 andthe tip 20 of electrode 35 is interchanged: the slide piece 36 is nowarranged in the center of spark-gap electrode tip 20 (actually formed bytwo electrode tips 20).

All publications mentioned in this specification are herein incorporatedby reference. While the invention has been illustrated by a descriptionof various embodiments and while these embodiments have been describedin considerable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. The invention in its broader aspects istherefore not limited to the specific details, representative methods,and illustrative examples shown and described. Accordingly, departuresmay be made from such details without departing from the spirit or scopeof applicant's general inventive concept.

The invention claimed is:
 1. A lightning current transfer arrangementfor providing electrical contact between first and second parts of awind turbine that are rotatable relative to each other, the lightningcurrent transfer arrangement comprising: a current transfer unitelectrically coupled to the first part, and an electrically conductiveslideway electrically coupled to the second part, and rotatable relativeto the current transfer unit, the current transfer unit comprising aslider elastically biased towards the slideway, wherein the slidercomprises: a slide piece forming a first current path between the firstand second parts through contact with the slideway, the slide pieceformed of a conductive plastic material and configured to conductelectrostatic charge between the first and second parts, and a metalelectrode rigidly coupled with the slide piece and configured to form asecond current path between the first and second parts through theslideway, wherein the metal electrode forms the second current path whena voltage drop through the slide piece exceeds a predetermined maximumvoltage drop.
 2. The lightning current transfer arrangement according toclaim 1, wherein the conductive plastic material comprises at least oneconductive additive.
 3. The lightning current transfer arrangementaccording to claim 2, wherein the at least one conductive additive is atleast one of carbon black, graphite, metal flakes, metal fibers,particles or fibers coated with metal, conductively coated minerals,conductively coated ceramics, and pulverised anthracite.
 4. Thelightning current transfer arrangement according to claim 1, wherein theconductive plastic material comprises one of an isolating matrix and aconductive matrix.
 5. The lightning current transfer arrangementaccording to claim 1, wherein the conducting plastic material exhibitsan electric conductivity from about 10⁻⁶ siemens per meter (S/m) toabout 10⁵ S/m.
 6. The lightning current transfer arrangement accordingto claim 1, wherein the metal electrode is arranged to also contact theslideway.
 7. The lightning current transfer arrangement according toclaim 6, wherein the slide piece and the metal electrode each comprise arespective sliding surface, wherein the sliding surfaces of the slidepiece and the metal electrode are co-planar.
 8. The lightning currenttransfer arrangement according to claim 7, wherein the sliding surfaceof the metal electrode is circumscribed by the sliding surface of theslide piece.
 9. The lightning current transfer arrangement according toclaim 1, wherein the metal electrode is arranged to move over theslideway at a distance, thereby forming a spark gap to the slideway. 10.The lightning current transfer arrangement according to claim 9, whereinthe slide piece comprises a sliding surface, wherein the metal electrodeis recessed from the sliding surface of the slide piece by a recessdistance, and wherein the slide piece follows the slideway duringmovement thereby maintaining the distance between the metal electrodeand the slideway at about the recess distance.
 11. The lightning currenttransfer arrangement according to claim 9, wherein the slide piece andthe metal electrode are electrically connected in parallel, wherein thesecond current path is formed in parallel to the first current path uponoccurrence of a spark bridging the spark gap.
 12. The lightning currenttransfer arrangement according to claim 11, wherein the slide pieceprovides an electric resistance in the first current path from about 10kiloohms (kΩ) to about 10 megaohms (MΩ).
 13. The lightning currenttransfer arrangement according to claim 1, further comprising: a secondslideway electrically coupled to the first part, the current transferunit further comprising a second slider providing electrical contact tothe second slideway, the second slider comprising a slide piececontacting the second slideway and an electrode oriented towards thesecond slideway.
 14. The lightning current transfer arrangementaccording to claim 13, wherein the wind turbine comprises at least onerotor blade and a nacelle, and wherein the lightning current transferarrangement is arranged between the rotor blade and the nacelle, andenables charges to be discharged from the rotor blade to ground via thenacelle.
 15. A wind turbine, comprising: a first part and a second partbeing rotatable relative to each other; and at least one lightningcurrent transfer arrangement arranged to provide electrical contactbetween the first and second parts, wherein the lightning currenttransfer arrangement comprises: a current transfer unit electricallycoupled to the first part, and an electrically conductive slidewayelectrically coupled to the second part, and rotatable relative to thecurrent transfer unit, the current transfer unit comprising a sliderelastically biased towards the slideway, wherein the slider comprises: aslide piece forming a first current path between the first and secondparts through contact with the slideway, the slide piece formed of aconductive plastic material and configured to conduct electrostaticcharge between the first and second parts, and a metal electrode rigidlycoupled with the slide piece and configured to form a second currentpath between the first and second parts through the slideway, whereinthe metal electrode forms the second current path when a voltage dropthrough the slide piece exceeds a predetermined maximum voltage drop.16. The wind turbine of claim 15, wherein the slide piece comprises asliding surface and contacts the slideway at the sliding surface,wherein the metal electrode is recessed from the sliding surface of theslide piece by a recess distance, and wherein the predetermined maximumvoltage drop is based on the recess distance.
 17. A lightning currenttransfer arrangement for providing electrical contact between first andsecond parts of a wind turbine that are rotatable relative to eachother, the lightning current transfer arrangement comprising: a currenttransfer unit electrically coupled to the first part, and anelectrically conductive slideway electrically coupled to the secondpart, and rotatable relative to the current transfer unit, the currenttransfer unit comprising a slider elastically biased towards theslideway, wherein the slider comprises: a slide piece forming a firstcurrent path between the first and second parts through contact with theslideway and configured to conduct charge of an electrostatic dischargebetween the first and second parts when the electrostatic dischargeresults in a voltage drop through the slide piece that is less than apredetermined maximum voltage drop, and a metal electrode rigidlycoupled with the slide piece and forming a second current path throughthe slideway, thereby conducting the charge of the electrostaticdischarge between the first and second parts, when the voltage dropexceeds the predetermined maximum voltage drop of the slide piece. 18.The lightning current transfer arrangement of claim 17, wherein theslide piece comprises a sliding surface and contacts the slideway at thesliding surface, wherein the metal electrode is recessed from thesliding surface of the slide piece by a recess distance, and wherein thepredetermined maximum voltage drop is based on the recess distance. 19.The lightning current transfer arrangement of claim 17, wherein theslide piece is further configured to limit an amount of current flowingthrough the slide piece when the electrostatic discharge results in avoltage drop greater than the predetermined maximum voltage drop. 20.The lightning current transfer arrangement of claim 19, wherein theelectrostatic discharge comprises a lightning strike on one of the firstpart and the second part.